Peptides and proteins, by virtue of their conformational versatility and functional specificity, have been used in treating a host of diseases including diabetes, hemophilia, cancer, cardiovascular disorders, infectious diseases and arthritis (Russell C. S. & Clarke L. A. Clin Gent 55(6):389 (1999); Ryffel B. Biomed environ Sci 10:65(1997); Koths K. Curr Opin Biotechnol 6:681 (1995); Buckel P. Trends Pharmacol Sci 17:450 (1996)). Presently, more than two thirds of the approved biotech medicines are systemic protein drugs. With recent advances in the field of functional genomics, proteomics and genetic engineering, an increasing number of protein drugs are entering the biopharmaceutical market.
Originally, protein drugs were purified from animal tissues or human serum. Protein-based pharmaceuticals have gone through several stages of improvement to reach the current state of clinical application. For example, the biopharmaceutical industry now uses genetically engineered yeast and bacteria to manufacture recombinant human proteins (Scopes R. K. Biotechnol Appl Biochem 23:197 (1996)). This groundbreaking technology has overcome the health risk and shortages that plagued the first generation of protein drugs, and has consequently improved the therapeutic value of proteins. However, despite these advances, broad usage of proteins as therapeutics is still hampered by difficulties in purifying recombinant proteins in active forms and the high cost of manufacturing procedures (Berthold W. & Walter J. Biologicals 22:135(1994); Scopes R. K. Biotechnol Appl Biochem 23:197 (1996)). Additionally, protein drugs face barriers to their entry into the body. When taken orally, they are susceptible to break down by enzymes in the gastrointestinal tract (Wang W. J Drug Target 4:195 (1996); Woodley J. F. Crit Rev Ther Drug Carrier Syst 11:61 (1994)).
Other routes of protein delivery explored include infusion pumps (Bremer et. al., Pharm Biotechnol 10:239 (1997)) transdermal delivery (Burkoth T. L. Crit Rev Ther Drug Carrier Syst 16:331 (1999)), microencapsulation (Cleland J. L. Pharma Biotechnol 10:1 (1997)) and inhalation (Gonda I. J Pharm Sci 89:940 (2000)). Currently, subcutaneous and intravenous administration by needle injection is the route of choice for delivering protein therapeutics. Unfortunately, this mode of delivery is less than ideal because protein concentrations often are not maintained within a therapeutic range or provide appropriate delivery kinetics. Furthermore, effective treatment with protein drugs usually requires frequent needle injections that can cause local reactions and discomfort, hence resulting in poor patient compliance (Jorgensen J. T. J Pediatr Endocrinol 7:175(1994)). These and other factors limit the therapeutic application of many drugs and ultimately hinder their commercial potential. Therefore, it is axiomatic to identify new delivery methods for protein therapeutics.
Insertion of genes encoding specific therapeutic proteins into cells of the body has been used to solve the aforementioned delivery problems in treating diseases. This methodology is referred to as gene therapy and it promises to be the new direction in protein delivery. By this approach, cells in the body can be transformed into ‘bioreactors’, manufacturing sufficient quantities of therapeutic proteins and hence eliminating the need for frequent needle injections. Currently, gene therapy can be categorized into two general approaches (Drew J. & Martin L-A. In: Lemoine N. R. (ed) Understanding Gene Therapy. Springer-Verlag, New York, Chp. 1: pp 1-10 (1999)).
In the first approach, referred to as in vivo gene therapy, a gene is introduced in a form that allows its absorption by cells located within the living host. For example, a therapeutic gene is packaged into the genome of viruses such as retrovirus, adeno-associated virus or adenovirus. The recombinant virus containing the therapeutic gene is then introduced into a living organism and allowed to infect cells within the organism. Through the infection process, the virus incorporates its genome containing the therapeutic genes into the genomic structure of the host cell. As a result, the infected cell expresses the therapeutic gene.
The second approach involves in vitro transfer of genetic material to cells removed from the host organism. Following successful incorporation of a gene into the cell's genome, the transformed cells are implanted back into the host. This gene transfer method is referred to as ex vivo gene therapy.
Both in vivo and ex vivo gene therapy offer physicians the power to add or modify specific genes resulting in disease cure (Friedmann T. In: Friedmann T (ed) The Development of Human Gene Therapy. Cold Spring Harbor Laboratory Press, Cold Spring Harbor. Chp 1:pp 1-20 (1999)). Clinical applications of this technology are being studied in a wide range of diseases, including cancer, cardiovascular disorders, metabolic diseases, neurodegenerative disorders, immune disorders and other genetic or acquired diseases ((Friedmann T. In: Friedmann T (ed) The Development of Human Gene Therapy. Cold Spring Harbor Laboratory Press, Cold Spring Harbor. Chp 1:pp 1-20 (1999); Drew J. & Martin L-A. In: Lemoine N. R. (ed) Understanding Gene Therapy. Springer-Verlag, New York, Chp. 1: pp 1-10 (1999)). Sustained therapeutic concentrations of numerous proteins have been achieved after stable introduction of genes that encode the proteins into cells by gene therapy methodologies. However, for some disorders, regulated delivery of the therapeutic protein is required. For example, insulin replacement therapy for diabetic patients ideally requires that the appropriate amount of insulin be delivered during meals. Likewise, optimal effectiveness of appetite suppressants may be achieved via meal-dependent release. Therefore, to deliver such therapeutic proteins, a release system triggered by a signal or stimuli, such as a meal, is optimal.
A particular disease well suited for timed delivery is diabetes mellitus, a debilitating metabolic disease caused by absent (type 1) or insufficient (type 2) insulin production from pancreatic β-cells (Unger, R.H. et al., Williams Textbook of Endocrinology Saunders, Philadelphia (1998)). β-cells are specialized endocrine cells that manufacture and store insulin for release following a meal (Rhodes, et. al. J. Cell Biol. 105:145(1987)) and insulin is a hormone that facilitates the transfer of glucose from the blood into tissues where it is needed. Patients with diabetes must frequently monitor blood glucose levels and many require multiple daily insulin injections to survive. However, such patients rarely attain ideal glucose levels by insulin injection (Turner, R. C. et al. JAMA 281:2005(1999)). Furthermore, prolonged elevation of insulin levels can result in detrimental side effects such as hypoglycemic shock and desensitization of the body's response to insulin. Consequently, diabetic patients still develop long-term complications, such as cardiovascular diseases, kidney disease, blindness, nerve damage and wound healing disorders (UK Prospective Diabetes Study (UKPDS) Group, Lancet 352, 837 (1998)).
Gene therapy represents a promising means to achieve physiologic delivery of therapeutic peptides such as insulin for the treatment of diabetes (Leibowitz, G. & Levine, F. Diabetes Rev. 7:124 (1999)). Surrogate cells that express the incorporated gene, process and store the encoded protein, and secrete insulin in regulated fashion therefore affords a treatment for diabetes. Controlling plasma insulin levels by coupling insulin production to changing nutrient requirements of the body also reduces the side effects associated with insulin injection. Accordingly, there is a need for controlled release of proteins to achieve effective treatment of diabetes and other diseases in humans. The present invention satisfies this need and provides related advantages.